(1) Field of the Invention
This invention relates to a novel calcia refractory, especially to an improved slaking-resistant calcia refractory which is best suited for use as material of basic refractory brick.
(2) Description of the Prior Art
Heretofore, magnesia, magnesia chromia and magnesia-alumina basic refractories have been popularly used as lining material for steel-making furnaces such as electric furnaces and converters, rotary cement kilns, etc., but because of the problems with these refractories such as voluminous energy consumption in the production process or possible environmental pollution due to use of chromia, changing from these refractories to calcia refractory has now become a matter of great concern. Because of its high slaking resistance, calcia has been noticed with great interest for its utilization as refractory material and attempts are being made to lower the firing temperature of this material from the viewpoint of energy saving.
As prior art for development of slaking-resistant calcia refractory, U.S. Pat. No. 2,916,389 discloses a method in which iron oxide is blended as slaking inhibitor, in an amount of 1-10% by weight, 1% by weight or less of silicon dioxide and 2-5% by weight of magnesium oxide and the blend is fired at a relatively low temperature, that is, 1,427.degree.-1,649.degree. C. However, further studies by this inventor revealed the fact that while blending of iron oxide is effective for promoting sintering and slaking resistance of the calcia refractory, it involves the possibility to reduce thermal shock resistance and hot strength of the refractory due to formation of low-melting matrices such as 3(Ca,Fe)O.(Fe, Al).sub.2 O.sub.3.3SiO.sub.2 and dicalcium ferrite (2CaO.Fe.sub.2 O.sub.3), so that it is desirable to confine the iron oxide content in the blend to the lowest possible level. It was also disclosed that when such iron oxide-containing calcia refractory is used for the parts contacting the molten steel, for example, bricks for ladle for making ingots or electric furnace hearth, there results greater wettability with the molten steel than the refractory not containing iron oxide, and also the specific gravity of the particles tends to increase, so that when these particles are mixed in the molten steel, they become more reluctant to float up and separate than the particles not containing iron oxide and tend to become inclusions in the oxide system. So, the inventors have conducted more minute studies on the relationship between the iron oxide content which exerts an influence on slaking resistance of calcia refractory and the contents of silicon dioxide, aluminum oxide and magnesium oxide. As a result, the fact was disclosed that the slaking resistance is reduced as the iron oxide content decreases, and in the case of the refractory preparations after firing at 1,400.degree. C. in which the iron oxide content was less than 0.45% by weight, their percent slaking retention after one-hour dealing in an autoclave after 5 atm. was excessively lessened, and particularly in the refractories containing no iron oxide after firing at 1,400.degree. C., the percent slaking retention in the autoclave was zero. It was noted, however, that the refractories fired at 1,500.degree. C. showed a very high percent slaking retention if the iron oxide content is properly adjusted.
In case such blend is fired in a rotary kiln or such, the firing efficiency is excessively lowered if the blend is fired in the form of powder, excepting the case where firing is performed by fluidized roasting, so that usually the powder is once shaped into pellets and then charged into the kiln for firing. However, this method still has some serious problems. The pellets maintain fairly high strength owing to the caking action of moisture before they are charged into the kiln, but when treated in the kiln, they are broken and pulverized in the course of drying and firing under mechanical impact in the kiln, resulting not only in a reduced yield of the fired grains but also heavy deposit of the pulverized particles on the furnace wall to cause ring trouble, or diminished furnace volume. In order to keep off such trouble, there is necessitated a caking agent that can give sufficient strength to the pellets in the temperature range used till formation of ceramic bonds in the firing process. The results of various studies on the organic and inorganic solid and liquid caking agents showed that the organic binders are detrimental to compaction of the clinker particles because such caking agents are scattered away during firing. Also, many of the inorganic caking agents lowered slaking resistance as they prevented the clinker particles from becoming compact or inhibited growth of the CaO crystal grains. Water may be used as an inorganic liquid caking agent but is not satisfactory. Calcia clinker not containing iron oxide is intolerably low in slaking resistance when the firing temperature is below 1,500.degree. C. This is attributable to a fairly wide variation of eutectic temperature of CaO-Al.sub.2 O.sub.3 -SiO.sub.2 or CaO-Al.sub.2 O.sub.3 -MgO-SiO.sub.2 system, depending on whether Fe.sub.2 O.sub.3 is present or absent. Therefore, in case of refractory blends not containing iron oxide, it is quite difficult to effect sintering at such low temperature as used in the above-mentioned method by merely adjusting the contents of the blended materials such as Al.sub. 2 O.sub.3 and MgO.